Known is an EGR apparatus which recirculates part of exhaust gas from an engine in a vehicle or the like to the engine to suppress generation of nitrogen oxides. Some of such EGR apparatuses are equipped with, midway of an exhaust gas recirculation line to the engine, an EGR cooler for cooling the exhaust gas since cooling the exhaust gas to be recirculated to the engine will drop the temperature of and reduce the volume of the exhaust gas to lower the combustion temperature in the engine without substantial decrease in output of the engine, thereby effectively suppressing generation of nitrogen oxides.
FIG. 1 is a sectional view showing an example of the EGR coolers in which reference numeral 1 denotes a cylindrical shell with axially opposite ends to which plates 2 are respectively fixed so as to close the ends of the shell 1. Penetratingly fixed to the respective plates 2 are opposite ends of a number of tubes 3 which extend axially within the shell 1.
The shell 1 is provided with an outer cooling water inlet pipe 4 in the vicinity of one end of the shell 1 and with an outer cooling water outlet pipe 5 in the vicinity of the other end of the shell 1 so that cooling water 9 is supplied via the cooling water inlet pipe 4 into the shell 1, flows outside of the tubes 3 and is discharged via the cooling water outlet pipe 5 from the shell 1.
The respective plates 2 have, on their sides away from the shell 1, bowl-shaped hoods 6 fixed to the respective plates 2 so as to enclose end faces of the plates. The one and the other hoods 6 provide central exhaust gas inlet and outlet 7 and 8, respectively, so that exhaust gas 10 from the engine enters via the exhaust gas inlet 7 into the one hood 6, is cooled during passage through the number of tubes 3 by means of heat exchange with cooling water 9 flowing outside of the tubes 3 and is discharged to the other hood 6 to be recirculated via the exhaust gas outlet 8 to the engine.
In the figure, reference numeral 11 denotes a bypass outlet pipe arranged at a position diametrically opposed to the cooling water inlet pipe 4, part of the cooling water 9 being withdrawn through the bypass outlet pipe 11 so as to prevent the cooling water 9 from stagnating at the position diametrically opposed to the cooling water inlet pipe 4.
Such conventional EGR cooler has poor heat exchange efficiency since the exhaust gas 10 flows straight in the tubes 3 and insufficiently contacts the inner peripheries of the tubes 3. Therefore, it has been proposed that an inner periphery of the tube 3 is formed with spiral protrusions to causes the exhaust gas 10 passing through the tube 3 to be whirled, thereby increasing contact frequency and contact distance of the exhaust gas 10 to the inner periphery of the tube 3 to enhance the heat exchange efficiency of the EGR cooler.
However, a design concept conventionally adopted for formation of a spiral protrusion on the inner periphery of the tube 3 is such that an inclination angle (to a plane perpendicular to an axis of the tube 3) of the spiral protrusion is minimized in merely focusing attention on initial performance value. It has been revealed, from experimental results by the inventor, that application of such design concept to a diesel engine from which the exhaust gas 10 with much sooty contents is discharged disturbs the flow of the exhaust gas 10 since the inclination angle of the spiral protrusion is small, resulting in an accumulation of soot within the tube 3 with a lapse of time, thus substantially lowering the heat exchange efficiency.
The invention was made in view of the above and has its object to provide an EGR cooler which can be satisfactorily applied to an diesel engine with no substantial lowering in performance.